In oil- and gas-field operations, polymer additives have been widely used for decades to enhance or modify the characteristics of the aqueous fluids used in well drilling, recovery and production applications.
One example of such use is for viscosity enhancement in water or other water-based (i.e., aqueous) fluids used for hydraulic fracturing treatments in subterranean well formations. Hydraulic fracturing creates fluid-conductive cracks or pathways in the subterranean rock formations in gas- and/or oil-producing zones, improving permeability of the desired gas and/or oil being recovered from the formation via the wellbore.
Many aqueous fracturing fluids utilize natural or synthetic viscosity-increasing polymers, some of which are categorized as gelling agents. Examples of such natural and synthetic polymer additives in fracturing fluids include guar, xanthan, cellulose derivatives and acrylamide-derived and acrylate-derived polymers and copolymers, and the like. Such polymers or copolymers may also be crosslinked to enhance their thickening or gelling properties.
The thickening or gel-forming additives allow the aqueous well fluid to carry a significant amount of a proppant, typically an inorganic solids propping agent like sand, into the fractures and fissures in the formation without premature settling of the proppant.
The polymer-thickened or polymer-gelled aqueous well fracturing fluid normally serves its intended purpose, propping open fractures and fissures in the oil- or gas-bearing formation via the suspended proppant, within a period of a few hours of its being introduced into the formation. A breaking agent or breaker, a component that is typically present in the original fracturing fluid formulation, is normally activated after the fracturing is completed to reduce (break) the viscosity of the gelled or thickened aqueous fluid, so as to facilitate recovery of the petroleum or hydrocarbon product. The breaking agents may be coated with various materials to delay their activity in the aqueous well fracturing fluid.
Commonly used or preferred breaking agents fall into two broad categories, enzymes (e.g., U.S. Pat. No. 7,431,087 of Sullivan et al.) and oxidizers. The second category includes persulfates (U.S. Pat. No. 3,167,510 of Alter; U.S. Pat. No. 4,552,675 of Brown et al.; U.S. Pat. No. 4,610,795 of Norris et al.), perborates (U.S. Pat. No. 3,024,191 of Jones), sodium and hydrogen peroxide (U.S. Pat. No. 2,483,936 of Roberts), and calcium, magnesium and zinc peroxides (U.S. Pat. No. 6,737,385 of Todd et al.).
Aqueous peracetic acid solutions have had limited use in commercial well drilling operations, and only a few uses in such applications are described or suggested in the patent literature.
Peracetic acid has been described as a bactericide for use in water, called flood waters, used in secondary oil recovery, in U.S. Pat. No. 3,329,610 of Kreuz et al., 1967). Another use for peracetic acid is well cleaning described in U.S. Pat. No. 3,470,959 of Kreuz et al., where peracetic acid is added to water (normally potable water) that is injected into so-called recharge wells that are used in conjunction with the removal of potable water from coastal underground potable water basins.
In well treatment operations involving the use of polymer thickeners, many oxidizing compounds have been mentioned in the patent literature as being potentially useful as viscosity breakers. Peracetic acid has been mentioned as one of many known oxidizers in many early and more recent patents as having potential usefulness as a viscosity breaker, e.g., U.S. Pat. No. 4,172,005 of DeMartino, U.S. Pat. No. 4,169,818 of DeMartino, U.S. Pat. No. 4,169,798 of DeMartino, U.S. Pat. No. 4,143,007 of DeMartino, U.S. Pat. No. 6,764,981 of Eoff et al., U.S. Pat. No. 7,159,656 of Eoff et al., U.S. Patent Publication No. 2004/0206502 of Reddy et al., U.S. Patent Publication No. 2006/0137874 of Dusterhoft et al. and U.S. Patent Publication No. 2006/0283592 of Sierra et al. None of these patents describes peracetic acid as being a preferred oxidizer for use as a viscosity breaker, much less exemplifies such use.
In another application in oil and gas well operations, hydrogen peroxide and per-acids such as peracetic acid are described as useful, in buffered solutions having H2O2 of 1-6 wt % or peracid concentrations of 1-15 wt %, for improving the permeability of wells by removing polymer deposits, in U.S. Pat. No. 7,156,178 of Rae et al.
The present invention provides an aqueous well treatment fluid containing a viscosity-enhancing polymer or copolymer in which a peracetic acid or other peracid is employed as a viscosity reducing agent, present in dilute concentrations, that is useful for effecting controlled and delayed viscosity-reduction activity in aqueous fluids intended for oil- and gas-field operations.